The present invention relates to a support for rolling bearings. As known, rolling bearings are mechanical devices in which a relative motion, e.g. between a rotating element and a stationary element or between two elements rotating at different speeds, takes place with the interposition of balls or rollers rolling between two tracks, one of which is directly formed on the rotating element or on a ring secured thereto and the other is formed on the stationary element or on a second ring secured thereto. Generally, the balls or rollers are kept spaced apart by a variously shaped cage capable of separating and retaining the balls or rollers.
In certain industrial applications, a resilient support has to be inserted between the bearings and the seat where they are housed. This resilient support aims at compensating for a possible alignment error of the bearings themselves and at preventing vibrations, generated also because of such an alignment error, from propagating from the rotating element to the structure of the machine where the element is mounted. Another source of vibrations is a non-uniform mass distribution around the geometrical rotation axis, resulting in centrifugal forces in the rotating element and, consequently, in the bearings thereof.
One of the applications in which the use of resilient supports is generally provided for is in rotary vacuum pumps, in particular turbomolecular vacuum pumps of the kind equipped with mechanical bearings. As known, rotary vacuum pumps are equipped with rotating shafts supported by magnetic bearings or by mechanical rolling bearings in which the shaft is made to rotate at a very high speed, typically in the range 20,000 to 90,000 rpm. If the pump is equipped with mechanical bearings, in order to prevent the vibrations of the shaft and the pumping rotor associated therewith from propagating to the pump structure, the rolling bearings are surrounded by annular supports of elastomeric material.
FIG. 1 shows an example of turbomolecular pump where rotor 101 has rotor disks 103 cooperating with stationary stator disks, secured with the pump casing (not shown), in order to pump gas between the inlet and exhaust ports of the pump. The rotor 101 is mounted on a rotating shaft 105 supported by ball bearings 107a and 107b, and the rotating shaft 105 is made to rotate by an electric motor 109 housed in a cavity 111 formed in pump basement 117.
As it can be clearly seen in the enlarged portion of FIG. 1, a resilient annular support 113 formed by one or more rings is provided between each ball bearing 107a, 107b and the respective seat 115 in pump basement 117. The provision of annular supports 113 is provided above all to the need of damping vibrations transmitted by the rotating pump elements to the body of the same pump and through the pump to the vacuum chamber.
In some particularly critical applications (for instance, in mass spectrometry), in which the vacuum pump is used jointly with very sophisticated measurement instruments, it is indispensable to prevent the vibrations of the pump rotor from being transmitted to the remaining structures and in particular to the instrumentation. A direct result of the damping of such vibrations is also the reduction of the overall pump noise.
The provision of annular supports 113 also assists in considerable reduction of the value of the fist critical speed (intended as the lowest rotation speed, associated with a modal form with substantially non-deformed rotor, or “rigid rotor”, at which the force transmitted to the bearings has a maximum), which in this manner is substantially lower than the nominal rotation speed of the pump, with a resulting effect of rotor self-balancing when such critical speed value is exceeded.
Further, the provision of the annular supports allows for compensating possible alignment errors related with the mechanical machining of the bearing seats. The alignment errors may be considerable and exceed the limits recommended by manufacturers for high speed precision bearings, such as those used for instance in turbomolecular pumps.
According to the prior art, the annular supports are resilient and are preferably made of elastomeric material, e.g. nitrile rubber. Yet, use of elastomeric supports entails serious drawbacks. The main drawback is due to the fact that the ball bearings mounted in vacuum pumps for operating correctly are generally subjected, to an axial preloading. For instance, in the case illustrated in FIG. 1, the preloading is exerted by spring 119. Now, the considerable axial friction existing between the external ring of the ball bearing and the elastomeric annular support can sensibly hinder the correct application of such preloading, especially as concerns bearing 107a, which is the farthest one from preloading spring 119 arranged in the pump basement.
In order to overcome such problem, there has been proposed to introduce metal inserts into the elastomeric annular support, in correspondence of the inner wall of the annular support, or to introduce a lubricant in correspondence of the inner wall. Such solutions, even if they reduce axial friction between the surface of the bearing and the support ring, cause other drawbacks. For instance, use of a lubricant introduces the risk of contaminating the vacuum chamber. Moreover, both solutions proposed above cause not only a reduction of axial friction, but also a reduction of tangential friction, which on the contrary has to be kept high to prevent the outer ring of the bearing from rotating relative to the annular support while the pump shaft is rotating.